Ferritic stainless steel sheet cover member and production method for ferritic stainless steel sheet

ABSTRACT

In a ferritic stainless steel sheet, the arithmetic average roughness Ra is 0.2 μm or more and 1.2 μm or less. In addition, the dull pattern transfer rate on the steel sheet surface is 15% or more and 70% or less. Furthermore, micropits with a depth of 0.5 μm or more and an open area of 10 μm 2  or more which are formed on the steel sheet surface have an existing density of 10.0 or less per 0.01 mm 2  and an open area ratio of 1.0% or less. In addition, a film formed on the steel sheet surface is constituted from an oxide containing SiO 2  as a main constituent, which oxide contains at least Si, N, Al, Mn, Cr, Fe, Nb, Ti and O as film-forming elements other than C, wherein the Si content is 10 at % or more, and the N content is 10 at % or less.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a U.S. national phase application under 35 U.S.C. § 371 ofInternational Patent Application No. PCT/JP2016/058375, filed Mar. 16,2016, and claims benefit of priority to Japanese Patent Application No.2015-076634, filed Apr. 3, 2015. The entire contents of theseapplications are hereby incorporated by reference.

FIELD OF TECHNOLOGY

The present invention relates to a ferritic stainless steel sheet whichis temper-rolled using a dull roll after finishing cold rolling andbright annealing, a cover member and a production method for a ferriticstainless steel sheet.

BACKGROUND

Austenitic stainless steel sheets typified by SUS304 and SUS316, andferritic stainless steel sheets typified by SUS430 have been mostly usedfor building exterior materials, building interior materials, andkitchen utensils, etc.

Then, in such uses, not only cleanability has been required to easilyremove various stains which are attached during producing products andconstruction, and various strains, fingerprints, etc. which are attachedduring daily use, but also antiglare properties are emphasized to makestains, fingerprints, handling scratches, etc. inconspicuous.

In addition, in members of precision instruments, electronic equipmentand the like, for e.g. hard disk drives (HDDs), densification andprocessing acceleration have been generally required.

In addition, materials used for HDD parts such as a rotary member, anarm member, a case member and a cover member are strictly controlledabout not only excellent corrosion resistance but also stains such asparticles (extraneous particles) and outgas.

Then, in the cleaning process when producing HDD parts, careful cleaningis carried out, for example ultrasonic cleaning using a fluorine-basedcleaning liquid, a weak alkaline cleaning liquid and ultrapure water andthe like after degreasing with a hydrocarbon.

In the cleaning process, vapor cleaning is also carried out as needed,and finally the rinsing process is carried out several times usingultrapure water to remove not only particles but also ionic substances.

Furthermore, in the cleaning process, cleaning is generally carried outunder clean circumstances with a cleanliness of Class 5 or higherprescribed by JIS B 9920 because fine dust existing in air is also astain source. It should be noted that Class 5 or higher prescribed byJIS B 9920 means a circumstance in which the number of 0.1 μm particlesis 100000 or less, the number of 0.2 μm particles is 23700 or less, thenumber of 0.3 μm particles is 10200 or less, the number of 0.5 μmparticles is 3520 or less, the number of 1 μm particles is 832 or lessand the number of 5 μm particles is 29 or less per m² of air.

For HDD parts produced through such cleaning process, normal steel, analuminum alloy, and a stainless steel, etc. are used, and are mostlyused with electroless Ni plating applied mainly for the purpose ofelevating corrosion resistance and improving cleanability.

Herein, HDD parts etc. are required to have not only corrosionresistance and cleanability but also a matte surface with antiglareproperties to make fingerprints and fine scratches inconspicuous.

In addition, as shown in FIG. 1, HDDs are provided with a sealing member3 such as a gasket or a rubber seal for a cover inner surface 2, whichis the inner side of a cover member 1, and the inside of HDDs and theoutside of HDDs are sealed for blockage with HDD parts assembled.

The sealing member 3 is fixed to a stainless steel constituting thecover member 1 with an adhesive, and thus the wettability of theadhesive and the stainless steel are important to maintain stablesealing properties. That is, it is required that the stainless steelconstituting the cover member 1 of HDDs have hydrophilicity on thesurface.

Then, as a stainless steel sheet for a cover member of precisioninstruments such as a HDD case, a stainless vibration damping steelsheet with excellent contamination resistance is known as described inPTL 1.

When a normal stainless steel sheet is annealed and pickled, a Crdepleted layer generated around the grain boundary near the surface byannealing is preferentially scarfed by pickling to form small grooves(microgrooves) along the grain boundary. When pickling is insufficient,the microgrooves become a factor to leave oil and to generate outgas. Inaddition, the microgrooves become a factor to reduce cleanabilitybecause dust is easily attached thereto.

In PTL 1, therefore, in order to prevent the occurrence of microgrooves,bright annealing or non-oxidation annealing is carried out as finishingannealing after cold rolling.

Then as a stainless steel sheet to which fine dust in air is not easilyattached, a stainless steel sheet with 10 or less pinholes with a sizeof above 0.25 mm² per 10 cm² on a temper rolled sheet surface obtainedby combining mechanical polishing, reduction annealing and temperrolling using a water-soluble lubricant is known as described in PTL 2.

Furthermore, as a stainless steel sheet with excellent stain resistanceand corrosion resistance, a stainless steel sheet with elevated stainresistance and corrosion resistance obtained by controlling a steelsheet surface to a predetermined surface roughness by bright annealingafter finishing rolling using a dull roll is known as described in PTL3.

As a stainless steel sheet with excellent contamination resistance,cleanability and antiglare properties, a steel sheet surface iscontrolled to a predetermined arithmetic average roughness by firsttemper rolling using a mirror roll after finishing annealing and thesecond temper rolling using a dull roll to elevate contaminationresistance, cleanability and antiglare properties as described in PTL 4.

Japanese Patent Publication No. 3956346

Japanese Laid-open Patent Publication No. 2001-20045

Japanese Patent Publication No. 3587180

Japanese Patent Publication No. 4226131

SUMMARY

However, it is believed that good cleanability to stains such as minuteparticles is not obtained only by applying bright annealing ornon-oxidation annealing as finishing annealing and omitting pickling asthe stainless steel sheet in PTL 1 described above.

In addition, the cleanability of the stainless steel sheet in PTL 2 isevaluated by a test in which a sample after completion of an exposuretest is only wiped once with a cloth immersed in a neutral detergent,and it is believed that good cleanability to stains such as minuteparticles is not obtained in the surface texture of the stainless steelsheet in PTL 2.

Herein, cleanability and antiglare properties are generallyinconsistent, and as a stainless steel sheet has more excellentantiglare properties, unevenness is larger on a steel sheet surface.Accordingly, stains are easily attached and furthermore attached stainsare not easily removed, and cleanability is deteriorated.

Therefore, in the stainless steel sheet in PTL 3, antiglare propertiescan be elevated, but cleanability is not investigated, and it isbelieved that good cleanability to stains such as minute particles isnot obtained.

In addition, it is believed that only by providing surface roughness,antiglare properties can be elevated but good cleanability to stainssuch as minute particles is not obtained as the stainless steel sheet inPTL 4.

The present invention has been made in view of such points, and anobject thereof is to provide a ferritic stainless steel sheet withexcellent cleanability, antiglare properties and hydrophilicity, a covermember and a production method for a ferritic stainless steel sheet.

The ferritic stainless steel sheet of the invention is a ferriticstainless steel sheet, which is temper-rolled using a dull roll afterfinishing cold rolling and bright annealing, wherein the arithmeticaverage roughness Ra in the direction perpendicular to the rollingdirection on the steel sheet surface is 0.2 μm or more and 1.2 μm orless, the transfer rate, which is the area rate of a part to which adull pattern is transferred on the steel sheet surface, is 15% or moreand 70% or less, micropits with a depth of 0.5 μm or more and an openarea of 10 μm² or more which are formed on the steel sheet surface havean existing density of 10.0 or less per 0.01 mm² on the steel sheetsurface and an open area ratio of 1.0% or less on the steel sheetsurface, and a film formed on the steel sheet surface is constitutedfrom an oxide containing SiO₂ as a main constituent, which oxidecontains at least Si, N, Al, Mn, Cr, Fe, Nb, Ti and O as film-formingelements other than C, wherein the Si content is 10 at % or more, andthe N content is 10 at % or less.

The ferritic stainless steel sheet can contain C: 0.15 mass % or less,Si: 0.1 mass % or more and 2.0 mass % or less, Cr: 10.0 mass % or moreand 32.0 mass % or less, and at least one of Nb: 0.01 mass % or more and0.8 mass % or less and Ti: 0.01 mass % or more and 0.5 mass % or less,and in which the rest includes Fe and inevitable impurities.

The ferritic stainless steel sheet can contain at least one of Mo: 0.2mass % or more and 5.0 mass % or less and Cu: 0.1 mass % or more and 3.0mass % or less.

The ferritic stainless steel sheet also can contain C: 0.15 mass % orless, Si: 0.1 mass % or more and 2.0 mass % or less, Mn: 2.0 mass % orless, P: 0.04 mass % or less, S: 0.03 mass % or less, Ni: 0.6 mass % orless, Cr: 11.0 mass % or more and 32.0 mass % or less, Mo: 0 mass % ormore and 3.0 mass % or less, Cu: 0 mass % or more and 1.0 mass % orless, Nb: 0 mass % or more and 1.0 mass % or less, Ti: 0 mass % or moreand 1.0 mass % or less, Al: 0 mass % or more and 0.12 mass % or less, N:0.025 mass % or less, and B: 0 mass % or more and 0.01 mass % or less,and in which the rest includes Fe and inevitable impurities.

A cover member of hard disk drives is formed from a ferritic stainlesssteel sheet according to any of the above examples.

The production method for a ferritic stainless steel sheet is aproduction method for a ferritic stainless steel sheet, in which a hotrolled steel sheet after hot rolling is subjected to at least finishingcold rolling, followed by bright annealing as finishing annealing, andtemper-rolled using a dull roll, wherein rolling is carried out at acold rolling reduction of 30% or more in finishing cold rolling, and arolling reduction of 15% or more and a rolling speed of 200 mm/min orless using a work roll with an arithmetic average roughness Ra of 0.3 μmor less at least in the final rolling pass, and the total cold rollingreduction until bright annealing is 70% or more.

The production method for a ferritic stainless steel sheet, where infinishing annealing, bright annealing is carried out in ahydrogen-nitrogen mixed gas atmosphere with a hydrogen ratio of 70 vol %or more under the condition that the dew point is −70° C. or higher and−50° C. or lower and the temperature is 800° C. or higher and 1100° C.or lower.

The production method for a ferritic stainless steel sheet where intemper rolling, rolling is carried out in a single pass or more using adull roll with a roll diameter of 500 mm or more and an arithmeticaverage roughness Ra of 1.0 μm or more and 3.5 μm or less at anelongation rate in a single pass of 0.5% or less, and the totalelongation rate is 0.2% or more and 1.4% or less.

According to the present invention, because the existing density andopen area ratio of micropits on a steel sheet surface are controlled,the arithmetic average roughness Ra on the steel sheet surface iscontrolled, and the dull pattern transfer rate on the steel sheetsurface is controlled, cleanability and antiglare properties can beelevated, and because the composition of a surface film formed on thesteel sheet surface is controlled, hydrophilicity can be elevated.

BRIEF DESCRIPTIONS OF DRAWINGS

FIG. 1 is a perspective view showing a cover member of HDDs.

DETAILED DESCRIPTION

The constitution in an embodiment of the present invention will now bedescribed in detail.

The ferritic stainless steel sheet in this embodiment is one which istemper-rolled using a dull roll after finishing cold rolling and brightannealing, and is appropriate as a material for a cover member etc. of,for example, hard disk drives (HDDs).

This ferritic stainless steel sheet is subjected to finishing coldrolling to obtain a predetermined surface texture. The surface filmstructure is controlled to hydrophilicity by bright annealing afterfinishing cold rolling, and furthermore, temper rolling is carried outto obtain a predetermined surface texture. Thus, cleanability is notreduced to the extent possible and antiglare properties are elevated.

First, the surface texture of a ferritic stainless steel sheet will bedescribed.

Cleanability showing the ease of removing stains attached onto a steelsheet surface is significantly affected by microscopic pits distributedon the steel sheet surface.

The pits are minute hollows on a steel sheet surface and mainly occurdue to fractures in the hot rolling process, gaps in grain boundaryoxidized parts, grain boundary corrosion parts, hollows generated inspace between different kinds of grains such as inclusions and carbides,falling traces of these grains, hollows due to insertion of metal grainsand other grains in the producing process, falling traces of remainingoxide scales, hollows due to entrainment of a rolling oil during coldrolling, fine surface scratches due to mismatches of cold rollingconditions, and working fractures due to inclusions during cold forming,and the like.

Among such pits, micropits with a depth of 0.5 μm or more and an openarea of 10 μm² or more easily act as trap sites for foreign substancessuch as fine stains, and are a major factor to inhibit cleanability.

Therefore, it is important to control the distribution of micropits on asteel sheet surface to elevate cleanability.

It should be noted that crater-shaped hollows themselves with a size ofseveral tens of μm to which a dull pattern is transferred by temperrolling with a dull roll do not correspond to micropits prescribed inthis embodiment, and a dull pattern is transferred to a micropit portionwhich has existed before temper rolling with a dull roll and a pit whichstill remains in the inner part of a crater, and a pit which is newlyopened in the inner part of a crater correspond thereto.

Then, when the existing density of micropits on a steel sheet surface ismore than 10.0 per 0.01 mm², and when the open area ratio of micropitson the steel sheet surface is more than 1.0%, micropits easily act astrap sites and cleanability is reduced.

Therefore, in order that a ferritic stainless steel sheet can securegood cleanability in the cleaning process carried out under cleancircumstances of Class 5 or higher prescribed in JIS B 9920, theexisting density of micropits on the steel sheet surface is 10.0 or lessper 0.01 mm² and the open area ratio of micropits on the steel sheetsurface is 1.0% or less.

It should be noted that the depth of pits is determined as the largestdepth of pits based on the average height of the pattern portion on theperimeter of pits. In addition, similarly, the depth of pits which existin the inner part of a crater to which a dull pattern is transferred isalso the largest depth of pits based on the average height of thepattern portion on the perimeter of pits.

The open area of a pit is the projected area of a part surrounded by themarginal portion of the pit with a steel sheet surface viewed in thedirection of thickness from the plane.

These depth and open area of a pit are preferably measured using a lasermicroscope and a white-light interference microscope, which can measurea surface form.

In addition, the measurement region by such measuring means ispreferably a total of 0.1 mm² or more in several visual fields randomlyselected from a steel sheet surface, and the depth and open area of pitsare measured, for example, by measurement in 20 visual fields or morewith a magnification of 1000, and furthermore the existing density andopen area ratio of micropits are calculated.

The number of micropits existing in a measurement region set in eachvisual field (including micropits in which a part of their openings isprojected from the boundary of the measurement region) is measured, andthe sum total of the measured numbers in each measurement region isdivided by the total area of all the measurement regions to calculatethe existing density of micropits as the number of micropits per 0.01mm².

In addition, the total of the open area of each micropit existing in ameasurement region set in each visual field (including, in a case wherea part of the opening of a micropit is projected from the boundary of ameasurement region, only the area of a part placed in the measurementregion) is calculated, and the sum total of all the open areas in eachmeasurement region is divided by the total area of each measurementregion to calculate the open area ratio of micropits.

Herein, a matte surface such as a dull pattern is appropriate as adesign for HDD members such as a cover member, and thus surfaceglossiness is reduced by temper rolling using a dull roll to provideantiglare properties. It is preferred that the standard of surfaceglossiness be glossiness prescribed by JIS Z 8741, i.e. the value at 20°is 400 or less.

As described above, when the arithmetic average roughness (Ra) of asteel sheet surface is less than 0.2 μm, a ferritic stainless steelsheet after temper rolling using a dull roll has high surface glossinessand a possibility that antiglare properties cannot be secured. On theother hand, when the unevenness of the steel sheet surface becomesgreater and Ra is above 1.2 μm, there is a possibility that cleanabilitywill be reduced. Therefore, in order to secure sufficient cleanabilityand antiglare properties, the Ra of a steel sheet surface is 0.2 μm ormore and 1.2 μm or less.

The arithmetic average roughness (Ra) is a measurement value prescribedby JIS B 0601, i.e. a measurement value in the direction perpendicularto the rolling direction.

The transfer rate, which is the area rate of a part to which a dullpattern is transferred by temper rolling on a steel sheet surface, isthe proportion of projected area of a part surrounded by the patternportion of a crater portion to which a dull pattern is transferred inthe total area of the steel sheet surface with the steel sheet surfaceviewed in the direction of thickness from the plane. For example, 20visual fields or more are observed with a magnification of 400 by anoptical microscope and the like, and a dull pattern transfer rate can becalculated by measuring the area rate of a crater portion to which adull pattern is transferred.

Herein, cleanability and antiglare properties are generallyinconsistent, and as the transfer rate on a steel sheet surface islower, cleanability can be elevated; however, the surface glossinessbecomes higher and antiglare properties are reduced. On the other hand,as the transfer rate is higher, the surface glossiness becomes lower andantiglare properties can be elevated; however, unevenness on the steelsheet surface becomes greater and cleanability is reduced.

More particularly, when the transfer rate is less than 15%, cleanabilitycan be elevated; however, antiglare properties are reduced, and stains,fingerprints and handling scratches are easily visible. On the otherhand, when the transfer rate is above 70%, antiglare properties can beelevated; however, the micropits occurrence in the inner part of acrater to which a dull pattern is transferred increases, and the openingof a micropit become greater, which causes a significant reduction incleanability.

Therefore, in order to secure both cleanability and antiglareproperties, the transfer rate on a steel sheet surface is 15% or moreand 70% or less.

In order to provide hydrophilicity for a ferritic stainless steel sheet,it is required that a surface film have a composition containing siliconoxide (SiO₂) as a main constituent, and as the amount of SiO₂ in thesurface film after bright annealing is larger, hydrophilicity can beelevated.

In addition, even when a surface film formed on a steel sheet surface isan oxidized film containing SiO₂ as a main constituent, the silicon (Si)content and the nitrogen (N) content in the oxidized film are importantto elevate hydrophilicity. That is, when an oxidized film contains, forexample, Si, nitrogen, aluminum (Al), manganese (Mn), chromium (Cr),iron (Fe), niobium (Nb), titanium (Ti) and oxygen (O) as film-formingelements other than carbon (C), the Si content and N content in theoxidized film are important.

More particularly, when the Si content in an oxidized film is less than10 at %, an oxidized film with a composition containing Cr and Fe oxidesas main constituents is produced, and hydrophilicity is not obtained.Therefore, the Si content in an oxidized film formed on a steel sheetsurface is 10 at % or more. In addition, the Si content in an oxidizedfilm is more preferably 15 at % or more.

In addition, it has been verified that when the N content in an oxidizedfilm is above 10 at %, hydrophilicity cannot be obtained. Therefore, theN content in an oxidized film formed on a steel sheet surface is 10 at %or less.

It should be noted that the analysis value of surface film compositionis a value calculated from a semi-quantitative analysis value based onthe integral area of each element spectrum by X-ray photoelectronspectroscopy.

The component composition of a ferritic stainless steel sheet will nowbe described.

The above ferritic stainless steel sheet contains 0.15 mass % or less ofC, 0.1 mass % or more and 2.0 mass % or less of Si, 10.0 mass % or moreand 32.0 mass % or less of Cr, and at least one of 0.01 mass % or moreand 0.8 mass % or less of Nb, and 0.01 mass % or more and 0.5 mass % orless of Ti, and the rest includes Fe and inevitable impurities.

In addition, a ferritic stainless steel sheet may have a compositioncontaining at least one of 0.2 mass % or more and 5.0 mass % or less ofmolybdenum (Mo) and 0.1 mass % or more and 3.0 mass % or less of copper(Cu) as needed.

C is a solid solution strengthening element, and when the Cconcentration is high, Cr carbides precipitated on the grain boundaryincrease. A Cr depleted layer with a lower Cr concentration is generatedaround Cr carbides, and starting from this part, micropits easily occur.In addition, micropits are opened and newly occur during temper rollingusing a dull roll, which causes the deterioration of cleanability. Then,when the C content is above 0.15%, cleanability is easily deteriorateddue to the Cr depleted layer. Therefore, the C content is 0.15 mass % orless.

Si is an alloy component which affects the amount of SiO₂ in a surfacefilm after bright annealing. That is, in order to provide hydrophilicityfor a ferritic stainless steel sheet as described above, it is preferredto increase the amount of SiO₂ in a surface film after bright annealing,but when the Si content in a ferritic stainless steel sheet, a rawsheet, is small, the proportion of Si in the surface film becomes lower,and an oxidized film containing SiO₂ as a main constituent is not easilyformed. Therefore, a higher Si content in steel of a raw sheet is morepreferred. More particularly, when the Si content is less than 0.1 mass%, there is a possibility that hydrophilicity cannot be sufficientlysecured. On the other hand, when the Si content is above 2.0 mass %,there is a possibility that cold workability will be reduced. Therefore,the Si content is 0.1 mass % or more and 2.0 mass % or less.

Cr is an alloy component effective to improve corrosion resistance, andwhen the Cr content is 10.0 mass % or more, the effect of improvingcorrosion resistance by adding Cr becomes remarkable. On the other hand,when Cr is contained in a large amount, above 32.0 mass %, there is apossibility that manufacturability will be deteriorated. Therefore, theCr content is 10.0 mass % or more and 32.0 mass % or less.

Nb coheres to C and N in steel as Nb(C, N) to generate precipitates, andsuppresses the generation of Cr carbides, which is one of the causes ofthe micropits occurrence, and thus is an important alloy component toelevate cleanability. Then, such effect becomes remarkable by adding Nbin an amount of 0.01 mass % or more. On the other hand, when Nb iscontained excessively, above 0.8 mass %, there is a possibility thatmanufacturability and workability will be deteriorated. Therefore, whenNb is contained, the Nb content is 0.01 mass % or more and 0.8 mass % orless.

Similarly to Nb, Ti coheres to C and N in steel as Ti(C, N) to generateprecipitates, and suppresses the generation of Cr carbides, which is oneof the causes of the micropits occurrence, and thus is an importantalloy component to elevate cleanability. Then, such effect becomesremarkable by adding Ti in an amount of 0.01 mass % or more. On theother hand, when Ti is contained excessively, above 0.5 mass %, there isa possibility that manufacturability and workability will bedeteriorated. Therefore, when Ti is contained, the Ti content is 0.01mass % or more and 0.5 mass % or less.

Mo and Cu are added as needed for the purpose of improving corrosionresistance. When Mo is contained, the effect of elevating corrosionresistance is shown by adding 0.2 mass % or more; however when Mo iscontained excessively, above 5.0 mass %, there is a possibility thattoughness will be reduced. In addition, when Cu is contained, the effectof elevating corrosion resistance is shown by adding 0.1 mass % or more;however, when Cu is contained excessively, above 3.0 mass %, there is apossibility that toughness will be reduced. Therefore, when Mo iscontained, the Mo content is 0.2 mass % or more and 5.0 mass % or less,and when Cu is contained, the Cu content is 0.1 mass % or more and 3.0mass % or less.

In addition to the above alloy components, other alloy components can bealso added as needed. For example, at least one of 2.0 mass % or more ofmanganese (Mn), 0.01 mass % or more and 0.5 mass % or less of zirconium(Zr), 0.05 mass % or less of yttrium (Y), 1.0 mass % or less of tungsten(W), 0.5 mass % or less of tin (Sn) and 1.0 mass % or less of cobalt(Co) and the like can be added to elevate corrosion resistance,workability and the like.

Furthermore, when considering a bad influence on characteristics by theabove alloy components, the phosphorus (P) content as impurities ispreferably controlled to 0.05 mass % or less, and the sulfur (S) contentis preferably controlled to 0.01 mass % or less.

It should be noted that the ferritic stainless steel sheet is notlimited to the above compositions, and may have compositionscorresponding to the types of ferritic stainless steel prescribed bye.g. JIS G 4305: 2005 and JIS G 4303: 2005. In addition to thesecompositions of ferritic stainless steel sheet, the ferritic stainlesssteel may contain 0.15 mass % or less of C, 0.1 mass % or more and 2mass % or more less of Si, 2.0 mass % or less of Mn, 0.04 mass % or lessof P, 0.03 mass % or less of S, 0.6 mass % or less of Ni, 11.0 mass % ormore and 32.0 mass % or less of Cr, 0 mass % or more and 3.0 mass % orless of Mo (including no addition), 0 mass % or more and 1.0 mass % orless of Cu (including no addition), 0 mass % or more and 1.0 mass % orless of Nb (including no addition), 0 mass % or more and 1.0 mass % orless of Ti (including no addition), 0 mass % or more and 0.12 mass % orless of Al (including no addition), 0.025 mass % or less of N (includingno addition), and 0 mass % or more and 0.01 mass % or less of boron (B(including no addition)), and the rest of the ferritic stainless steelsheet may include Fe and inevitable impurities.

A production method for the above ferritic stainless steel sheet willnow be described.

In order to produce a ferritic stainless steel sheet with excellentcleanability and antiglare properties, it is important that annealing,pickling, finishing cold rolling and bright annealing are successivelycarried out to produce a ferritic stainless steel raw sheet with a fewmicropits, smoothness and excellent cleanability, and this raw sheet istemper-rolled using a dull roll under a light pressure to provideantiglare properties with cleanability maintained as much as possible.

First, using a hot rolled steel sheet produced by a conventional methodas a starting material, relatively coarse extraneous substances such asmetal and scales are removed, in the annealing and pickling processesand the like.

Next, rolling is carried out at a sufficient rolling reduction infinishing cold rolling, and in the final stage (final pass) of thefinishing cold rolling, rolling is carried out using a work roll withhigh smoothness at a low velocity under the condition of high pressureto smooth hollows (falling traces) generated by pickling and hollows bygrain boundary corrosion to the extent possible. Simultaneously, hollowsderived from a hot rolled steel sheet and hollows such as falling tracesin the annealing and pickling processes are smoothened to the extentpossible by significantly increasing the total cold rolling reductionuntil bright annealing.

Furthermore, the formation of hollows due to surface oxidation isprevented by carrying out bright annealing as finishing annealing afterfinishing cold rolling, and besides the subsequent pickling is notrequired, which does not cause grain boundary corrosion by pickling, anda ferritic stainless steel raw sheet with excellent cleanability isproduced.

Then, in order to control a ferritic stainless steel raw sheet thusproduced to the above existing density and open area ratio of micropits,temper-rolling is carried out using a dull roll on a predeterminedcondition that the opening and occurrence of micropits can be suppressedto provide antiglare properties with cleanability maintained.

It should be noted that, when producing a ferritic stainless steelsheet, a method in which using a hot rolled steel sheet as a startingmaterial, bright annealing is carried out as finishing annealing atleast after finishing cold rolling, followed by temper rolling using adull roll is only needed. As a specific producing procedure, forexample, a ferritic stainless steel sheet can be produced from a hotrolled steel sheet by a procedure (i) in which processing is allowed toproceed in order of annealing, pickling, finishing cold rolling,finishing annealing (bright annealing) and temper rolling. In addition,as another procedure, a procedure (ii) in which processing is allowed toproceed from a hot rolled steel sheet in order of annealing, pickling,cold rolling, annealing, pickling, finishing cold rolling, finishingannealing (bright annealing) and temper rolling can be used.Furthermore, a procedure (iii) in which processing is allowed to proceedfrom a hot rolled steel sheet in order of annealing, pickling, firstcold rolling, first annealing, first pickling, second cold rolling,second annealing, second pickling, finishing cold rolling, finishingannealing (bright annealing) and temper rolling can be used. Inaddition, a procedure (iv) in which processing is allowed to proceedfrom a hot rolled steel sheet in order of annealing, pickling, coldrolling, bright annealing, finishing cold rolling, finishing annealing(bright annealing) and temper rolling can be used.

It should be noted that in the above procedures (i) to (iv), thegrinding process and the degreasing process can be added as needed, andthe finishing processes such as degreasing, a tension leveler andslitting can be applied to a sheet after the final temper rollingwithout affecting a surface texture.

Specific conditions in each process in the above production method willbe described.

A hot rolled steel sheet is a steel sheet which is only hot-rolledwithout cold rolling. This hot rolled steel sheet is one in which astainless steel is smelted, casted and hot rolled by a conventionalmethod, and subjected to hot rolling, annealing and pickling as needed.

Annealing and pickling are processing effective to remove coarse foreignsubstances such as metal and scales which are attached to a steel sheetsurface.

The annealing conditions can be suitably selected considering themanufacturability and characteristics of raw materials. In addition,either annealing method, batch-type annealing and continuous annealing,can be used without affecting the surface texture of a steel sheet, andcan be selected, for example, depending on its raw materials.

Pickling is carried out by combining neutral salts and acids such assulfuric acid, nitric acid, hydrofluoric acid and hydrochloric acid, andelectrolytic pickling can be also carried out.

Finishing cold rolling is cold rolling carried out immediately beforebright annealing after the final annealing, and the number of passes maybe once or several times. In addition, several kinds of rolling machinesuch as general Sendzimir mill and a mill for thin sheets can be used inorder. The cold rolling reduction of finishing cold rolling when usingdifferent rolling machines in order is the total cold rolling reductionof several rolling machines.

Such finishing cold rolling is an important process to determine thesurface texture of a ferritic stainless steel sheet. That is, in orderthat micropits will have predetermined existing density and open arearatio in finishing cold rolling, it is important to fully draw fallingtraces of foreign substances generated by pickling and hollows by grainboundary corrosion in finishing cold rolling.

Then, when the cold rolling reduction in finishing rolling is less than30%, there is a possibility that hollows cannot be fully drawn.Therefore, the cold rolling reduction in finishing cold rolling is 30%or more. It should be noted that the cold rolling reduction ispreferably 40% or more and further preferably 50% or more. In addition,the cold rolling reduction in finishing cold rolling is affected bymaterial deformation resistance and the ability of a cold rollingmachine used, and thus the upper limit thereof can be suitably selectedand is commonly 90% or less.

In addition, in finishing cold rolling, when a work roll with a rollsurface arithmetic average roughness Ra of above 0.3 μm is used at leastin the final rolling pass, and when the rolling reduction in the finalrolling pass is less than 15%, there is a possibility that the smoothingof a steel sheet surface will be insufficient and cleanability will bereduced. Therefore, it is required to use a work roll with a rollsurface arithmetic average roughness Ra of 0.3 μm or less at least inthe final rolling pass in finishing cold rolling, and furthermore it isrequired that the rolling reduction in this final rolling pass be 15% ormore.

Furthermore, when the rolling speed in the final rolling pass is above200 m/min, there is a possibility that the opening and occurrence ofmicropits will proceed by the entrainment of a rolling oil into a workroll and a steel sheet surface. Therefore, the rolling speed in thefinal rolling pass in finishing cold rolling is 200 m/min or less.

Herein, many surface defects generated during hot rolling are relativelydeep. In order to reduce micropits as many as possible, it is importantto increase the total cold rolling reduction until the bright annealingprocess and fully draw surface defects existing in a hot rolled steelsheet, a starting material. There is also a possibility that foreignsubstances buried around a steel sheet surface will fall by annealing,pickling of a hot rolled sheet and the like before cold rolling, and inorder to draw the falling traces, it is effective to increase the totalcold rolling reduction.

The total cold rolling reduction is the total rolling reduction of coldrolling in a series of processes until bright annealing when producing aferritic stainless steel sheet. For example, in the above procedure (i),it means the rolling reduction of finishing cold rolling, in the aboveprocedure (ii), it means the total rolling reduction of cold rolling andfinishing cold rolling, in the above procedure (iii), it means the totalrolling reduction of cold rolling 1, cold rolling 2 and finishing coldrolling, and in the above procedure (iv), it means the total rollingreduction of cold rolling and finishing cold rolling. More particularly,when the sheet thickness before the first cold rolling pass of a hotrolled steel sheet is h0 (mm) and the sheet thickness after the finalcold rolling pass is h1 (mm), the total cold rolling reduction isrepresented by ((h0−h1)/h0)*100(%).

Then, as a result of investigation, it has been found that, when thetotal cold rolling reduction, which is the total cold rolling reductionuntil bright annealing, is 70% or more, surface defects can beeffectively removed. Therefore, the total cold rolling reduction untilbright annealing is 70% or more. It should be noted that the total coldrolling reduction is affected by material deformation resistance and theability of a cold rolling machine used, and thus the upper limit thereofcan be suitably selected and is commonly 98% or less.

In order to maintain a surface texture obtained by such finishing coldrolling, i.e. a surface texture with a very few micropits, it isimportant that surface oxidation is prevented in finishing annealing andfurthermore the subsequent processes for removing oxide scales such aspickling and grinding can be omitted. Therefore, bright annealing iscarried out as finishing annealing in a reducing atmosphere.

Bright annealing is annealing in a reducing atmosphere, and ispreferably carried out on the condition of bright annealing processingapplied to the BA finish (JIS G 203: 2009, No. 4225).

In addition, in order to obtain an oxidized film structure withexcellent hydrophilicity in bright annealing, it is required to form anoxidized film containing SiO₂ as a main constituent by annealing in ahydrogen and nitrogen mixed gas atmosphere with a hydrogen ratio of 70vol % or more.

Then, when the dew point is above −50° C. during annealing, an oxidizedfilm becomes an oxide containing Cr and Fe as main constituents, andfurthermore the oxidized film becomes too thick, which easily causecoloration due to interference color (temper color). On the other hand,when the dew point is lower than −70° C., Si is easily reduced, and thusan oxidized film containing SiO₂ as a main constituent is not easilyformed, and furthermore Al is easily thickened in the film. In addition,when temperature during annealing is lower than 800° C. and above 1100°C., Si is not sufficiently thickened in an oxidized film, and anoxidized film containing SiO₂ as a main constituent is not easilyformed. Therefore, bright annealing is carried out in ahydrogen-nitrogen mixed gas atmosphere with a hydrogen ratio of 70 vol %or more under the condition that the dew point is −70° C. or higher and−50° C. or lower and the temperature is 800° C. or higher and 1100° C.or lower.

A dull pattern is transferred to a steel sheet surface by temper rollingusing a dull roll as a work roll after bright annealing to provideantiglare properties with cleanability maintained.

In such temper rolling, in order to suppress the opening and occurrenceof micropits in the inner part of a crater to which a dull pattern istransferred, and provide antiglare properties without deterioration ofcleanability, it is important to control the dull rolling condition.

Then, when the diameter of a dull roll is smaller than 500 mm, there isa possibility that stress will be applied to a crater portion to which adull pattern is transferred more than necessary to develop the openingand occurrence of micropits in the inner part of a crater.

In addition, when the surface roughness of a dull roll has an arithmeticaverage roughness Ra of 1.0 μm or more and 3.5 μm or less, antiglareproperties can be provided and cleanability is not easily reduced.

With respect to the pass condition of temper rolling, when theelongation rate per pass is above 0.5%, there is a possibility that theopening and occurrence of micropits in the inner part of a crater willproceed. In addition, even when the total elongation rate is identical,temper rolling in more passes by a plurality of passes is preferredbecause the opening and occurrence of micropits in the inner part of acrater to which a dull pattern is transferred can be suppressed.

Furthermore, when the total elongation rate, which is the totalelongation rate in temper rolling, is less than 0.2%, there is apossibility that antiglare properties cannot be sufficiently provided,and when the total elongation rate is above 1.4%, there is a possibilitythat cleanability will be reduced.

Therefore, in temper rolling, it is preferred that the diameter of adull roll be 500 mm or more, the surface roughness of this dull rollhave an arithmetic average roughness Ra of 1.0 μm or more and 3.5 μm orless, the elongation rate in a single pass be 0.5% or less, and thetotal elongation rate be 0.2% or more and 1.4% or less.

In such temper rolling, a lubricant blended with e.g. additives for thepurpose of e.g. rust prevention can be used. In addition, a work rollsurface can be wiped with e.g. a wiper using a cleaning solution toremove foreign substances.

The action and effect of the above embodiment will now be described.

According to the above ferritic stainless steel sheet, the existingdensity of micropits which are the cause of attachment of stains to asteel sheet surface is 10.0 or less per 0.01 mm², and the open arearatio on a steel sheet surface is 1.0% or less, and thus the trap sitesof e.g. particles are not easily generated, and cleanability can beelevated.

In addition, the arithmetic average roughness Ra on a steel sheetsurface is 0.2 μm or more and 1.2 μm or less, and furthermore the dullpattern transfer rate on a steel sheet surface is 15% or more and 70% orless, and thus cleanability can be maintained, and furthermore antiglareproperties can be elevated.

Furthermore, a surface film formed on a steel sheet surface isconstituted from an oxide containing SiO₂ as a main constituent whichoxide has a composition containing Si, N, Al, Mn, Cr, Fe, Nb, Ti and Oas film-forming elements other than C, wherein the Si content is 10 at %or more and the Ni content is 10 at % or less, and thus hydrophilicitycan be elevated.

Therefore, the surface texture and surface film on a ferritic stainlesssteel sheet are controlled as described above, and thus cleanability,antiglare properties and hydrophilicity can be elevated.

In addition, a ferritic stainless steel sheet has excellentcleanability, antiglare properties and hydrophilicity, and thus can beappropriately used as a cover member of HDDs.

According to the above production method for a ferritic stainless steelsheet, rolling is carried out at a cold rolling reduction of 30% or morein finishing cold rolling and a rolling speed of 200 mm/min or less sothat the rolling reduction will be 15% or more using a work roll with anarithmetic average roughness Ra of 0.3 μm or less at least in the finalrolling pass in finishing cold rolling, and thus the micropitsoccurrence can be suppressed, and cleanability can be elevated bysmoothening a steel sheet surface.

Furthermore, the total cold rolling reduction until bright annealing is70% or more, and thus surface defects are effectively removed, themicropits occurrence can be suppressed and cleanability can be elevated.

By bright annealing after finishing cold rolling in a hydrogen-nitrogenmixed gas atmosphere with a hydrogen ratio of 70 vol % or more under thecondition that the dew point is −70° C. or higher and −50° C. or lowerand the temperature is 800° C. or higher and 1100° C. or lower, anoxidized film containing SiO₂ as a main constituent is formed as asurface film on a steel sheet surface and thus hydrophilicity can beelevated.

By temper rolling after bright annealing using a dull roll with adiameter of 500 mm or more and an arithmetic average roughness Ra of 1.0μm or more and 3.5 μm or less at an elongation rate in a single pass of0.5% or less so that the total elongation rate is 0.2% or more and 1.4%or less, cleanability is not reduced to the extent possible andantiglare properties can be elevated.

EXAMPLES

Examples and Comparative Examples will now be described.

Example 1

First, a stainless steel with a chemical composition shown in Table 1was smelted by an electric furnace, a converter and the VOD process,followed by continuous casting to obtain a slab.

TABLE 1 Steel Alloy component content (mass %) Category type C Si Mn CrTi Nb Others Examples A 0.07 0.57 0.14 16.3 0.02 <0.01 B 0.01 0.45 0.1517.2 <0.01 0.34 C 0.01 0.56 0.14 18.2 <0.01 0.41 Cu: 0.5 Comparative D0.16 0.52 0.14 16.4 0.01 <0.01 Examples E 0.01 0.05 0.20 16.3 <0.01 0.20F 0.07 0.48 0.13 16.5 <0.01 <0.01

Next, a continuous casting slab was hot-rolled by a common method toobtain a hot rolled steel sheet. In addition, using the hot rolled steelsheet as a starting material, each process was allowed to proceed in theorder mentioned in the above procedure (ii) or procedure (iii), andfurthermore a dull roll was used in the temper rolling process to obtaina temper rolled material with a sheet thickness of 0.3 to 1.5 mm, whichwas used as a test piece in each Example and each Comparative Example.The production condition of each of these Examples and ComparativeExamples will be described in Table 2.

It should be noted that, in the steel type B and the steel type E inTable 2, each process was carried out by the procedure (ii), and in theother steel types, each process was carried out by the procedure (iii).In addition, all Examples used a work roll with a Ra of 0.3 μm or lessin finishing cold rolling, the rolling reduction in the final rollingpass was 15% or more, and the rolling speed in the final rolling passwas 200 mm/min or less. In all Examples, furthermore, bright annealingwas carried out in an atmosphere in which hydrogen is 75 to 100 mass %and the rest is nitrogen.

TABLE 2 Finishing Finishing cold annealing Cold rolling (BA) rollingcondition condition condition Rolling Roll roughness Rolling Coldrolling Hydrogen gas Sample Total rolling reduction in final passreduction in speed in final Temperature concentration Dew number Steeltype reduction (%) (%) (μm) final pass (%) pass (m/min) (° C.) (%) point(° C.) Electroless — — — — — — — — Ni plating A-1 A 93.3 70.0 0.1 15.0100 900 75 −60 A-2 93.3 70.0 0.1 25.0 200 900 75 −60 A-3 66.7 50.0 0.115.0 100 900 75 −60 A-4 72.2 23.1 0.1 15.0 150 900 75 −60 A-5 72.2 44.40.5 15.0 150 900 75 −60 A-6 72.2 44.4 0.1 12.0 250 900 75 −60 B-1 B 86.172.2 0.1 15.0 150 1020 75 −60 B-2 86.1 72.2 0.1 15.0 150 1020 75 −60 B-386.1 72.2 0.1 15.0 150 1020 75 −60 B-4 86.1 72.2 0.1 15.0 150 1020 75−60 B-5 86.1 72.2 0.1 15.0 150 1020 75 −60 C-1 C 88.9 75.0 0.1 15.0 601020 75 −60 C-2 88.9 75.0 0.1 15.0 60 1020 75 −60 C-3 88.9 75.0 0.1 15.060 1020 75 −60 C-4 88.9 75.0 0.1 15.0 60 750 75 −50 C-5 88.9 75.0 0.115.0 60 1020 60 −50 C-6 88.9 75.0 0.1 15.0 60 1020 75 −45 D-1 D 88.975.0 0.1 15.0 150 950 75 −60 D-2 88.9 75.0 0.1 15.0 150 950 75 −60 E-1 E93.3 85.0 0.1 15.0 150 950 75 −60 E-2 93.3 70.0 0.1 15.0 150 950 75 −60F-1 F 88.9 50.0 0.1 15.0 150 950 75 −60 F-2 88.9 50.0 0.1 15.0 150 95075 −60 Dull roll temper rolling condition Elongation rate Sample Rolldiameter Dull roll in single pass Number of Total elongation Final sheetnumber Steel type (mm) roughness (μm) (%) passes rate (%) thickness (mm)Category Electroless — — — — — — — Ni plating A-1 A 760 1.7 0.30 1 0.300.3 Example A-2 760 2.3 0.30 3 0.90 0.5 Example A-3 760 1.7 0.30 4 1.201.0 Comparative Example A-4 760 1.7 0.30 3 0.90 1.0 Comparative ExampleA-5 760 1.7 0.30 3 0.90 0.5 Comparative Example A-6 760 1.7 0.30 3 0.900.5 Comparative Example B-1 B 760 1.8 0.30 2 0.60 0.5 Example B-2 4501.8 0.30 2 0.60 0.5 Comparative Example B-3 760 3.9 0.30 2 0.60 0.5Comparative Example B-4 760 2.0 0.05 2 0.10 0.5 Comparative Example B-5760 2.0 0.50 3 1.50 0.5 Comparative Example C-1 C 760 1.8 0.30 1 0.300.5 Example C-2 760 1.8 0.10 1 0.10 0.5 Comparative Example C-3 760 1.80.65 1 0.65 0.5 Comparative Example C-4 760 1.8 0.30 3 0.90 0.5Comparative Example C-5 760 1.8 0.30 3 0.90 0.5 Comparative Example C-6760 1.8 0.30 3 0.90 0.5 Comparative Example D-1 D 760 1.5 0.30 3 0.900.5 Comparative Example D-2 760 1.5 0.30 3 0.90 0.5 Comparative ExampleE-1 E 760 1.7 0.20 2 0.40 0.3 Comparative Example E-2 760 1.7 0.30 20.60 0.3 Comparative Example F-1 F 760 2.3 0.20 3 0.60 0.5 ComparativeExample F-2 760 2.3 0.40 3 1.20 0.5 Comparative Example

Using each test piece shown in Table 2, measurements about cleanability,antiglare properties and hydrophilicity were carried out. Moreparticularly, the measurement of arithmetic average roughness on a steelsheet surface, the transfer rate measurement, the measurement ofmicropits on a steel sheet surface, the measurement of surfaceglossiness, the surface film measurement, the measurement ofwettability, and the evaluation of cleanability were carried out.

It should be noted that similarly the cleanability of an electroless Niplating material, which is mostly used for HDD parts, was also measuredas a control material for evaluating cleanability as shown in Table 2.

In the measurement of arithmetic average roughness on a steel sheetsurface, a 50 mm square sample cut from each test piece was subjected toultrasonic cleaning using acetone, and the arithmetic average roughnessRa was then measured in a method in accordance with JIS B 0601. Itshould be noted that this arithmetic average roughness was measuredthree times in the direction perpendicular to the rolling direction andthe average value was calculated and evaluated.

In the transfer rate measurement, a 50 mm square sample cut from eachtest piece was subjected to ultrasonic cleaning using acetone, and asteel sheet surface was then observed by an optical microscope tocalculate the transfer rate, which is the area rate of a crater portionto which a dull pattern is transferred. It should be noted that a steelsheet surface was observed with a magnification of 400, and the numberof observed visual fields was 20, and the average value of all themeasurement values was calculated and evaluated.

In the measurement of micropits, a 50 mm square sample cut from eachtest piece was subjected to ultrasonic cleaning using acetone, and asteel sheet surface was then observed by a laser microscope to calculatethe existing density and open area ratio of micropits with a depth of0.5 μm or more and an open area of 10 μm² or more. It should be notedthat a steel sheet surface was observed with a magnification of 1000,and the number of visual fields was 10, and the total area ofmeasurement regions was 0.1 mm².

In the measurement of surface glossiness, a 50 mm square sample cut fromeach test piece was subjected to ultrasonic cleaning using acetone, andthe surface glossiness (20°) was then measured in a method in accordancewith JIS Z 8741. It should be noted that the surface glossiness wasmeasured three times in each direction, the direction parallel to therolling direction and the direction perpendicular to the rollingdirection, to calculate the average value, and a sample with 400 or lesswas evaluated as one with low surface glossiness and excellent antiglareproperties.

In the surface film measurement, the proportion of Si element wasobtained from the integrated intensity of each element peak on theoutermost surface of an oxidized film in each sample by X-rayphotoelectron spectroscopy.

In the measurement of wettability, a 50 mm square sample cut from eachtest piece was subjected to ultrasonic cleaning using acetone, and thecontact angle of a 0.1 ml droplet of ion exchanged water was thenmeasured by a sessile drop method. A sample with a contact angle of 50°or less was evaluated as one with excellent wettability.

In the evaluation of cleanability, a 50 mm square sample cut from eachtest piece was subjected to a cleaning operation in the followingprocedure to obtain a specimen for measuring surface cleanliness. Itshould be noted that the processes after acetone degreasing, which is acleaning operation, and all the processes for measuring surfacecleanliness were carried out under clean circumstances of Class 5prescribed in JIS B 9920.

In the cleaning operation of samples, first, degreasing is carried outby ultrasonic cleaning using acetone. This degreased sample wassubjected to ultrasonic cleaning using a fluorine-based cleaning liquid,vapor cleaning and vacuum drying. After that, the sample was subjectedto ultrasonic cleaning using a weak alkaline detergent, rinsed byimmersion in ultrapure water, pulled up at a low speed and dried withwarm air to obtain a specimen for measuring surface cleanliness.

The surface cleanliness was measured using an LPC (Liquid ParticleCounter) device as follows.

First, in order to immerse a specimen for measuring cleanliness,ultrapure water was put into a beaker, which was set to the LPC device,and the number of particles existing in the ultrapure water and theparticle size distribution were measured. From the measurement data ofthis ultrapure water, the number of particles with a particle diameterof 0.3 μm or more was calculated, and this calculated value was used asthe number of particles before the specimen was immersed (a blankmeasurement value).

Next, a specimen for measuring cleanliness was immersed in the beaker ofultrapure water and was subjected to ultrasonic cleaning for a fixedtime to extract particles attached to the specimen surface intoultrapure water. After that, the number of particles existing in thisultrapure water and the particle size distribution were measured by theLPC device to calculate the number of particles with a particle diameterof 0.3 μm or more.

Then, the difference between this calculated value and the blankmeasurement value was used as the number of particles extracted from aspecimen for measuring cleanliness. It should be noted that the numberof particles and the particle size distribution were measured threetimes or more using the same solution by the LPC device, and the averagevalue was used as a measurement value. In addition, using 3 samples ofthe same type of specimen, measurement was carried out at the number oftests n=3, and the average value was used as the number of particlesattached to and remaining in a specimen for measuring cleanliness.Furthermore, the number of attached particles (the number of particlesattached to the surface) per unit area on a steel sheet surface wascalculated from the value of particles. Then, when the number ofattached particles was 1000/cm² or less, cleanability was evaluated asgood.

The measurement results about the cleanability, antiglare properties andhydrophilicity are shown in Table 3.

TABLE 3 Contact Micropits Number of particles Sample Transfer Siproportion angle Number Open area attached to surface number Steel typerate (%) Ra (μm) Glossiness in film (at %) (degree) (micropits/0.01 mm²)ratio (%) (particles/cm²) Category Electroless Ni — — — — 1 0.1 400 —plating A-1 A 16 0.26 380 12 21 3 0.10 500 Example A-2 45 0.72 43 15 406 0.50 700 Example A-3 68 1.10 35 16 30 30 1.50 2100 Comparative ExampleA-4 42 0.70 34 16 41 32 1.70 3000 Comparative Example A-5 41 0.70 40 1638 18 1.60 2000 Comparative Example A-6 44 0.71 46 16 48 15 1.80 1500Comparative Example B-1 B 33 0.52 120 11 20 6 0.20 700 Example B-2 280.42 130 12 30 20 2.30 1800 Comparative Example B-3 39 0.62 80 12 25 484.20 5600 Comparative Example B-4 10 0.14 450 12 41 4 0.30 600Comparative Example B-5 75 1.22 30 12 38 18 1.60 2300 ComparativeExample C-1 C 17 0.25 320 12 40 6 0.20 700 Example C-2 5 0.08 500 12 452 0.20 500 Comparative Example C-3 30 0.52 200 12 21 19 1.80 2300Comparative Example C-4 41 0.63 40 8 61 4 0.30 600 Comparative ExampleC-5 41 0.64 40 2 54 5 0.30 500 Comparative Example C-6 41 0.65 40 5 57 80.30 700 Comparative Example D-1 D 39 0.62 45 12 38 30 3.00 2300Comparative Example D-2 45 0.70 51 12 39 35 3.20 3000 ComparativeExample E-1 E 28 0.50 120 8 65 8 0.30 700 Comparative Example E-2 330.52 170 8 70 6 0.20 700 Comparative Example F-1 F 30 0.54 80 12 45 434.00 4300 Comparative Example F-2 55 0.84 48 12 60 52 4.00 5800Comparative Example

As shown in Table 3, in all Examples, the existing density of micropitswas 10.0 or less per 0.01 mm² and the open area ratio of micropits was1.0% or less. In addition, a stainless steel sheet, in which thearithmetic average roughness in the direction perpendicular to therolling direction on a steel sheet surface was 0.2 to 1.2 μm and thedull pattern transfer rate is 15 to 70%, was obtained.

In addition, in all the stainless steel sheets in Examples, the numberof attached particles in a specimen for measuring surface cleanlinesswas 1000 particles/cm² or less, which was equally low compared to thatof an electroless Ni plating material, a control material for evaluatingcleanability.

Furthermore, all the stainless steel sheets in Examples had surfaceglossiness lower than the standard, which means good antiglareproperties, and furthermore had a contact angle smaller than thestandard, which means good hydrophilicity.

Therefore, it can be evaluated that all Examples have a surface statewith cleanability, antiglare properties and hydrophilicity appropriateas, for e.g. a cover member of HDDs, even on an unprocessed stainlesssteel sheet surface.

Example 2

On the surface of some samples made in Example 1, a gasket wasinjection-molded, and the adhesion of an adhesive between the stainlesssteel and the gasket was evaluated.

When injection molding a gasket, a modified olefin resin adhesive wasfirst applied to a sample surface in advance.

In addition, a gasket was injection-molded using a styrene thermoplasticelastomer compound at injection speed of 0.3 mm/sec, an injectionpressure of 30 MPa and a cycle time of 30 seconds by an injectionmolding machine to adhere to the sample surface.

Then, a test for adhesive properties was carried out as follows.

In the test for adhesive properties, penetration peeling with about 1 mmwas formed on the adhesive surface of gasket formed on a sample, and anSUS wire was allowed to pass through a part in which this penetrationpeeling was formed. A vertical tensile load was applied thereto and whenpeeled length was expanded to about 10 mm, its load was measured.

When this measured peeling load was 100 kPa or more, adhesive propertieswere evaluated as good, and when the peeling load was less than 100 kPa,adhesive properties were evaluated as poor. The samples, the adhesiveproperties of which were evaluated, and the results are shown in Table4.

TABLE 4 Sample Number Steel Type Adhesion of Adhesive Category A-1 AGood Example B-1 B Good Example C-1 C Good Example C-5 Poor ComparativeExample C-6 Poor Comparative Example E-2 E Poor Comparative Example

Examples A-1, B-1 and C-1, which had good cleanability, antiglareproperties and wettability in Example 1, all had good adhesiveproperties.

On the other hand, Comparative Examples C-5, C-6 and E-2, which had poorwettability in Example 1, all had poor adhesive properties.

From the above, it can be evaluated that the ferritic stainless steelsheet according to the present invention has a surface state withcleanability, antiglare properties and hydrophilicity appropriate as acover member of HDDs.

The present invention can be used when producing parts for precisioninstruments and electronic equipment and the like, for e.g. a covermember of hard disk drives (HDDs).

1. A ferritic stainless steel sheet, which is temper-rolled using a dullroll after finishing cold rolling and bright annealing, comprising: anarithmetic average roughness Ra in a direction perpendicular to arolling direction on a steel sheet surface is 0.2 μm or more and 1.2 μmor less, a transfer rate, which is an area rate of a part to which adull pattern is transferred on the steel sheet surface, is 15% or moreand 70% or less, micropits with a depth of 0.5 μm or more and an openarea of 10 μm² or more which are formed on the steel sheet surface havean existing density of 10.0 or less per 0.01 mm² on the steel sheetsurface and an open area ratio of 1.0% or less on the steel sheetsurface, and a film formed on the steel sheet surface is constitutedfrom an oxide containing SiO₂ as a main constituent, which oxidecontains at least Si, N, Al, Mn, Cr, Fe, Nb, Ti and O as film-formingelements other than C, wherein the Si content is 10 at % or more, andthe N content is 10 at % or less.
 2. The ferritic stainless steel sheetaccording to claim 1, containing C: 0.15 mass % or less, Si: 0.1 mass %or more and 2.0 mass % or less, Cr: 10.0 mass % or more and 32.0 mass %or less, and at least one of Nb: 0.01 mass % or more and 0.8 mass % orless and Ti: 0.01 mass % or more and 0.5 mass % or less, and the restincluding Fe and inevitable impurities.
 3. The ferritic stainless steelsheet according to claim 2, containing at least one of Mo: 0.2 mass % ormore and 5.0 mass % or less and Cu: 0.1 mass % or more and 3.0 mass % orless.
 4. The ferritic stainless steel sheet according to claim 1,containing C: 0.15 mass % or less, Si: 0.1 mass % or more and 2.0 mass %or less, Mn: 2.0 mass % or less, P: 0.04 mass % or less, S: 0.03 mass %or less, Ni: 0.6 mass % or less, Cr: 11.0 mass % or more and 32.0 mass %or less, Mo: 0 mass % or more and 3.0 mass % or less, Cu: 0 mass % ormore and 1.0 mass % or less, Nb: 0 mass % or more and 1.0 mass % orless, Ti: 0 mass % or more and 1.0 mass % or less, Al: 0 mass % or moreand 0.12 mass % or less, N: 0.025 mass % or less and B: 0 mass % or moreand 0.01 mass % or less, and the rest including Fe and inevitableimpurities.
 5. A cover member of hard disk drives, the cover memberbeing formed from a ferritic stainless steel sheet according to any oneof claim
 1. 6. A production method for a ferritic stainless steel sheet,comprising: finishing cold rolling a hot rolled steel sheet after hotrolling, bright annealing, following the finishing cold rolling, asfinishing annealing, and temper rolling using a dull roll, whereinrolling is carried out at a cold rolling reduction of 30% or more infinishing cold rolling, and a rolling reduction of 15% or more and arolling speed of 200 mm/min or less using a work roll with an arithmeticaverage roughness Ra of 0.3 μm or less at least in the final rollingpass, and the total cold rolling reduction until bright annealing is 70%or more.
 7. The production method for a ferritic stainless steel sheetaccording to claim 6, wherein, in finishing annealing, bright annealingis carried out in a hydrogen-nitrogen mixed gas atmosphere with ahydrogen ratio of 70 vol % or more under the condition that the dewpoint is −70° C. or higher and −50° C. or lower and the temperature is800° C. or higher and 1100° C. or lower.
 8. The production method for aferritic stainless steel sheet according to claim 6, wherein in temperrolling, rolling is carried out in a single pass or more using a dullroll with a roll diameter of 500 mm or more and an arithmetic averageroughness Ra of 1.0 μm or more and 3.5 μm or less at an elongation ratein a single pass of 0.5% or less, and the total elongation rate is 0.2%or more and 1.4% or less.